Explanation/Reference:
The Data Pump Transportable Tablespace method can be used only if the on-premises platform is little endian, and the database character sets of your on-premises database and Oracle Database Cloud Service database are compatible.
Note: If the job you are attaching to was initially started using an encryption password, then when you attach to the job you must again enter the ENCRYPTION_PASSWORD parameter on the command line to re-specify that password. The only exception to this is if the job was initially started with the ENCRYPTION=ENCRYPTED_COLUMNS_ONLY parameter. In that case, the encryption password is not needed when attaching to the job.
References: https://docs.oracle.com/en/cloud/paas/database-dbaas-cloud/csdbi/mig-11g-11g.html